1. Field of the Invention
The present invention relates to a contact for a vacuum interrupter which has excellent characteristics of high withstand voltage, large current durability and low chopping current.
2. Description of the Prior Arts
The important characteristics of a contact for a vacuum interrupter include:
(1) high interrupting property of a current interrupter;
(2) high withstand voltage;
(3) small contact resistance;
(4) low melt-bonding force;
(5) low erosion of a contact; and
(6) small chopping current.
It has been difficult to obtain a contact which is practically used and has all satisfactory characteristics. Accordingly, it has been considered to use a contact which has certain important characteristics even though the contact has inferior characteristics for other features depending upon its usage for a vacuum interrupter.
For example, a copper-bismuth alloy (Cu-Bi) has been mainly used for a contact for a vacuum interrupter. According to our experience, a contact made of the Cu-Bi alloy has the following disadvantages. A contact made of the Cu-Bi alloy containing less than 0.5 wt. % of Bi has large chopping current whereas a contact made of the Cu-Bi alloy containing more than 0.5 wt. % of Bi has relatively low withstand voltage.
When the chopping current is large, there is a possibility to cause abnormal voltage between contacts. When the withstand voltage is low, the contact can not be used in a high voltage circuit.
On the other hand, it has been well-known that a contact made of tungsten has high withstand voltage in vacuum. However, tungsten is a heat resistant metal whereby thermionic emission characteristic is high and metal vapor pressure is low. Accordingly, high chopping property can not be expected for a contact made of tungsten itself. Tungsten has low conductivity whereby contact resistance between the contacts is great, whereby it is difficult to use tungsten itself for a circuit interrupter having large current capacity in view of elevation of temperature.
It is effective to add copper in order to overcome the disadvantages of tungsten. A copper-tungsten alloy can be a substrate for a contact having high withstand voltage. However, the chopping current of the copper-tungsten alloy is affected by the interrupting properties of copper itself, tungsten itself and the interface between copper and tungsten and also a ratio of copper to tungsten.
The chopping current is increased depending upon increasing a ratio of copper to tungsten (wt. %) in the mixture of copper and tungsten. However, the chopping current is smaller depending upon decreasing diameter of particles of tungsten in the case of a constant ratio of copper to tungsten (wt. %).
The phenomenon is caused by the following reason.
The conductivity of copper is remarkably greater than that of tungsten whereby clear shunt phenomenon is resulted in the small current region near the chopping current. Accordingly, the current is mainly passed through copper parts.
When the skeleton of tungsten is formed by smaller particles of tungsten powder, the copper component is finely and uniformly dispersed to reduce the part of pool of the copper component.
The part of the copper component dispersed finely and uniformly has smaller heat capacity to give higher elevation of temperature by a constant current and to reduce the chopping current in comparison with the part of pool of the copper component.